Authorities were quick to deny that the green pool posed a risk to divers' health, but that actually depends on why the water changed colour.

The possible culprits are: a sudden algae bloom; a change in pool alkalinity; or a chemical reaction in the water. How do these cause a change in the water colour?

Blooming algae

Algal blooms, caused by a sudden proliferation of microscopic algae or cyanobacteria, are found in ponds, rivers and seas the world over. But they are certainly not expected in Olympic swimming pools.

Phenomena such as “red tides” and their beautiful night-time bioluminescence occur when conditions are perfect for rapid algae growth.

There’s no single triggering factor for this growth, but large amounts of rain (which cause nutrients to be flushed into the water), followed by sunny and warm weather as well as calm waters, are thought to be contributing environmental factors.

Olympic officials have suggested that the remarkable change in pool colour could be a result of the sudden growth of an algal species. While small amounts of algae are present in swimming pools, they are not detectable by the human eye and the chemicals used to treat the water usually keep them at bay.

Warm weather conditions compounded by low winds may have contributed to an algae outbreak, but it seems more likely that a chemical imbalance or broken filter system is the underlying cause of the change in colour.

A not-so-basic explanation

Some news sources have reported that a “decrease in alkalinity” is responsible for the fluctuations in the colour of the diving and water polo pools. One of the tanks connected to the pool is said to have run out of one of the chemicals used to moderate the pH of the water.

An ideal pH for a swimming pool is slightly alkaline (just over pH 7) to match the pH of our eyes and mucus membranes and hence avoid irritation.

The chemicals responsible for maintaining a safe pH also influence the multitude of chemical reactions occurring in the swimming pool.

Chlorine, for example, is the most widely known disinfectant used in swimming pools. But chlorine’s ability to kill pathogens is reduced when the pH of the pool is altered.

A reduction in alkalinity could have triggered the suspected algal bloom.

Rio’s water polo pool also turned green. Reuters/Kai Pfaffenbach

Chemical relay reaction

A cocktail of chemicals is necessary to keep pools, which are basically large bodies of stagnant water, safe for swimming and, in particular, to kill harmful bacteria or parasites such as E. Coli and Giardia lamblia.

Chemicals such as chlorine aren’t just reactive to pathogens, though. They also react with chemicals introduced to the water by the swimmers. Olympian perspiration, urine and sun tan lotion can all react with the chemicals added to the water to produce byproducts.

The addition of extra chlorine to the chameleon-like water polo pool has been blamed for the stinging eyes of the Australian water polo players. But one of the aforementioned byproducts may actually be responsible.

Urine contains a lot of a nitrogen-rich molecule called urea, which reacts with chlorine to form an irritant called trichloramine. Even the characteristic smell of a swimming pool is not actually chlorine but rather a collection of similar byproducts.

Gold, silver or copper?

The presence of metal ions can also lead to changes of colour in aqueous environments such as swimming pools. Some have suggested that copper or other metals from water pipes could be responsible for the dramatic green of the diving pool.

The simplest complex that copper forms with water is a blue solution, hexaaquacopper (II), where a copper ion is completely surrounded by six molecules of water. If the water is displaced by other molecules, such as chloride, colour changes can result.

Different metals form different coloured solutions depending on their oxidation state and the nature of the molecules coordinated to the metal. Some combinations result in green solutions. But none of the analyses from the Rio pools have mentioned metal complexes so far.

No need to adjust your sets

Both Brazilian experts and FINA have checked the water quality and determined that it poses no risk to athletes. Olympic officials will be hoping the colour of the pool changes back to blue and that the focus returns to the feats of those in the water rather than the water itself.